1. Field of the Invention
The present invention generally relates to a control method and system of a fuel cell system and, more particularly, to a control method and system of a fuel cell system, which vary hydrogen supply pressure when stopping power generation of a fuel cell.
2. Description of the Related Art
A fuel cell system applicable to a hydrogen fuel cell vehicle, a type of eco-friendly vehicle, is composed of a fuel cell stack that generates electric power from an electrochemical reaction of reactant gases; a hydrogen supplying system configured to supply hydrogen as fuel to a fuel cell stack; an air supplying system configured to supply gas including oxygen as an oxidant in electrochemical reactions; and a heat and water management system configured to manage water and maintain an optimal fuel cell stack temperature for driving by emitting heat, which is a by-product of the electrochemical reactions therein.
FIG. 1 is an exemplary view illustrating an overall fuel cell system. As shown in FIG. 1, the fuel cell system 100 includes a fuel cell stack 10, a fuel cell load device 20, an air blower 30, a humidifier 40, air shutoff valves 35 and 45 in an inlet and outlet, a drain valve 42, a purge valve 44, a water trap 50, a hydrogen recirculation device 55, a hydrogen supply valve 57, a radiator 60, and a thermostat 65. The air shutoff valves 35 and 45 in the inlet and outlet may prevent inflow of air to the fuel cell stack after a fuel cell vehicle shuts down. The drain valve 42 is disposed within a hydrogen exhaust line to remove water generated at an anode, and the purge valve 44 adjusts hydrogen concentrations at the anode and exhausts hydrogen to an air outlet to dilute the air.
Further, the fuel cell load device 20 that reduces the voltage of the fuel cell stack 10 to drain the voltage is connected to the fuel cell stack 10 to remove oxygen within the fuel cell stack 10 while the fuel cell vehicle is shutting down and after the fuel cell vehicle has shut down. The oxygen flowing into the fuel cell stack 10 is removed with residual hydrogen that collects at the anode as the fuel cell load device 20 consumes current. However, insufficient hydrogen may preclude complete consumption of the oxygen, and thus a Wakeup technique is used to periodically supply hydrogen to the anode. When stopping power generation of a fuel cell (FC STOP), the amount of hydrogen crossover from the anode to a cathode should be decreased through anode pressure control to comply with exhaust gas regulations. Accordingly, a method for decreasing the amount of hydrogen crossover is required.